Sound insulators and sound absorbers have heretofore been used to insulate noises. As the sound absorber, it is common to use a soft non-woven fabric or a molded foamed plastic article which absorbs an acoustic wave to a high degree. Such a sound absorber is not rigid and not expected to exhibit sound insulating properties. On the other hand, as the sound insulator, it is common to use a member having high density and high rigidity which hardly vibrates by an acoustic wave. To insulate noises, such a sound insulator insulates an acoustic noise by reflection, not by absorption. Therefore, generally, such a sound insulator is not expected to exhibit sound absorbing properties.
The above-mentioned conventional sound absorber or sound insulator cannot exhibit both sound-absorbing properties and sound-insulating properties. Therefore, in order to provide a member having both sound absorbing and insulating properties, a troublesome process such as laminating a sound absorber and a sound insulator is required, which makes an entire production complicated. If an attempt is made to ensure both sound-absorbing and insulating properties by laminating a sound absorber and a sound insulator, any one of product properties, such as heat resistance, rigidity, light weight, and shape, may be deteriorated. To avoid this, it is required to optimize a material constituting a sound absorber and a sound insulator which are to be laminated, which may result in a time-consuming work of material selection.
Therefore, Patent Document 1 and Patent Document 2 propose a sound absorber using a porous molded article formed of a plurality of foamed resin particles integrated into one piece and a molded article of resin particles having voids communicating with one another. But the sound absorber and the molded article of resin particles described in these documents are a porous molded article having no skin layer. Therefore, these have such disadvantages that they cannot be used suitably in structural bodies due to insufficient strength and are not expected to exhibit sound insulating properties. In addition, the above-mentioned sound absorber and molded article of resin particles also have a disadvantage that it cannot absorb a plurality of frequency bands selectively although they can absorb a single frequency band by controlling the particle size.
In Patent Documents 3 to 5, the applicant of the invention proposes a technique of ensuring both sound absorbing properties and sound insulating properties by using a material having skin layers and a void layer, with one of the skin layers being perforated. These materials are excellent in sound absorbing properties and sound insulating properties. But in this technique, perforation is performed during the post-molding process. To stabilize sound absorbing properties, it is required to remove chips accumulated in holes. In this respect, this technique is not satisfactory in productivity.    Patent Document 1: JP-A-H07-168577    Patent Document 2: JP-A-H10-329220    Patent Document 3: JP-A-2000-52371    Patent Document 4: JP-A-2003-337588    Patent Document 5: WO03/91987
According to application, a sound absorber may require to selectively absorb a sound in a specific frequency band. For example, a car engine sound contains both an uncomfortable noise and a comfortable sound. If an attempt is made to absorb an engine sound using the conventional sound absorber, there is a problem that both an uncomfortable noise and a comfortable sound are absorbed since the conventional sound absorber cannot select an absorbable frequency band. Accordingly, there is a demand for a sound absorber which effectively absorbs an uncomfortable noise and does not absorb a comfortable sound.
The invention has been made in view of the above-mentioned problems, and is directed to provide a process for producing a sound absorber excellent in productivity and quality stability and capable of absorbing only a noise in a specific frequency band, a sound absorber, and a sound absorbing structure.